Your mother in law sounds like she really is having some problems concerning her son's tragic death.
At the same time you are experiencing enormous strains with her never ending presence.
Have you or your husband tried talking to her at all?
Does she have any friends or other family, could you try talking to her husband? Does he have any idea what his wife's behaviour is doing to you>
I know from your point of view how incredibly distressing you must be finding it, but there really must be something deeply worrying your mother in law for her to behave like this.
Maybe for the moment you could try to limit the visits to just that, no overnight stays, but try to explain to your mother in law and father in law your reasoning behind the change.
It's really hard as you want to encourage a lovely relationship with grandparents, but this one sounds a little unnerving at the moment and it could end in a massive argument as by the sounds of your post, both your husband and yourself are really feeling the strain of the relationship.
I really don't know what else to suggest, but you need to try and get back to some form of healthier relationshio that is not so all consuming on the part of your mother in law and your daughter, but without obviously causing any distress to your daughter as she clearly adores her grandmother.
Please hang on in there. If you could try to talk to your father in law or get your husband to talk to him and be completely open and honest with him as you have been in your post here then maybe he could help in the situation as well.
